Coating compositions which enable the formation of a resin coating on a metal surface by bringing said metal surface into contact with an acidic composition which includes an organic film-forming resin are known as "autodepositing" or "autodeposition" compositions and are usually water-based. Such compositions are disclosed in Japanese Patent S47-17630, Japanese Examined Patent S52-21006, Japanese Examined Patent S54-13435 and Japanese Unexamined Laid-Open Patent Application S61-168673.
The defining characteristic of autodepositing waterbased coating compositions is that by contacting a suitable metal material having a clean surface with the coating composition, a resin coating film which increases in thickness or weight as immersion time increases forms spontaneously as a result of chemical reaction between the metal and the coating composition. (Metal ions dissolved from the metal surface are believe to interact with the resin particles and to be deposited together with them onto the metal surface.) Thus, a resin coating film can be beneficially formed on said metal surface without the need for external electromotive force, as is required in electrodeposition. However, in some cases the corrosion resistance and adhesion of an autodeposited resin coating film is not entirely satisfactory, and various means have been disclosed for improving these properties. Such means include chemical treatments of wet autodeposited coating films before they are dried. (Such chemical treatments are known by various names in the art; the most common such name is probably "final rinse", which is well-established but often a misnomer, because there may be other subsequent water rinses. Other names are "after-treatment", "post-treatment", "reaction rinse", "chemical rinse", and the one preferred herein, "modifying rinse".) The following citations exemplify such treatments.
In Japanese Examined Patent S53-15093 and U.S. Pat. No. 3,795,546 it is disclosed that by bringing a coated object into contact with an aqueous solution containing a hexavalent chromium compound and polyacrylate after immersion in an autodepositing aqueous coating composition and before hot drying the coated resin film, the corrosion resistance of the coating film after drying is raised.
In Japanese Unexamined Laid-Open Patent Application S51-30245 a method is disclosed for preventing poor appearance, such as blistering and cracking and the like, of autodeposited resin coating films after hot drying, by bringing the said resin coating film before drying into contact with an aqueous solution containing from 10-100 grams per liter (hereinafter usually abbreviated as "g/L") of a water-soluble solvent such as an alcohol, ketone, alcohol ester, ketone ester, ketone ether, or ester ether.
In Japanese Unexamined Laid-Open Patent Application S52-68240 a method is disclosed for raising the corrosion resistance of an autodeposited resin coating film formed on a coated object after hot drying by bringing said resin coating film before drying into contact with an aqueous solution or aqueous dispersion containing from 5-100 g/L in total of one or more substances selected from the group consisting of nitrogen-containing organic compounds such as amines, carboxylic acid amine salts, amino acids, melamine and amides, for example.
In Japanese Unexamined Laid-Open Patent Application H3-505841 a method is disclosed for raising the post-drying adhesion and corrosion resistance of an autodeposited resin coating film formed on a coated object by bringing said resin coating film, before hot drying, into contact with an aqueous solution of an alkaline substance.
Similarly, Japanese Unexamined Laid-Open Patent Application H5-186889 discloses a method for raising the post-drying adhesion and corrosion resistance of an autodeposited resin coating film formed on a coated object by bringing the said resin coating film before hot drying into contact with an aqueous solution with a pH from 7 to 11 containing from 0.05 to 5 percent by weight (hereinafter usually abbreviated as "wt %") of anions derived from an acid selected from 1,1-diphosphonic acid, citric acid, succinic acid and oxalic acid.
Because coating by autodeposition is generally performed by immersion, in many cases the coating liquid is prone to being initially retained in, and later to sag or run in the vicinity of, specific structural features, such as bolt holes when the coated material is to be bolted in place. In such instances, a consistent coating film appearance is not usually obtained, because there is a variation in gloss between the portions where sagging occurs and other portions. When an autodeposited resin coating film is the last or only coating employed on the coated object, this variation in gloss considerably affects the quality of the appearance of the coated object. The pre-drying modifying rinses of autodeposited resin coating films known from the art described previously are beneficial for raising the corrosion resistance of said resin films and for preventing blistering and cracking of the resin film, but have not been found to have any benefit when it comes to preventing variation in gloss. In recent years the painting of metal surfaces is being required to give greater added value, so that high corrosion resistance, high adhesion and also a uniform appearance are required, and an improvement in this appearance would be desirable.